ICONE15-10275 Effect of Correlations of Component Failures and Cross-Connections of EDGs on Seismically Induced Core Damages of a Multi-Unit Site

Abstract

Aiming at proposing effective applications of seismic probabilistic safety assessment (PSA) for design and risk management of nuclear facilities, a preliminary seismic PSA study was conducted for a multi-unit site to examine core damage frequency (CDF) and core damage sequences with consideration of the effect of correlations of component failures. In addition, the effectiveness of an accident management measure, namely, cross-connections of emergency diesel generators (EDGs) between adjacent units was also examined. Twin BWR-5 units of the same design were hypothesized to be located at the same site in this study and the CDF as well as the accident sequences of this two-unit site was analyzed by using SECOM2, a system reliability analysis code for seismic PSA. The results showed that the calculated CDF depends on the assumptions on component correlations and when the rules for assigning correlation coefficients defined in the NUREG-1150 program was adopted, the CDF of a single unit, the CDF of this two-unit site (the frequency of core damages of at least one unit at this site) and the frequency of simultaneous core damages of both units increased by factors of about 1.3, 1.2 and 2.3, respectively. In addition, it might be possible that the simultaneous core damages of both units was caused by pairs of different accident sequences in addition to pairs of the same sequences. When cross-connections of EDGs between two units were available, the CDF of a single unit, the CDF of this two-unit site as well as the frequency of simultaneous core damages of both units decreased. In addition, the CDF of this two-unit site was smaller than the CDF of a single unit site. It seemed that cross-connections of EDGs would be beneficial for a multi-unit site if the rules for assigning correlation coefficients defined in NUREG-1150 program were reasonable.